Smoke detectors are useful to detect fire conditions within supervised environmental regions. Some examples of these detectors are photo and ionization detectors. Adding gas sensors to smoke detectors can improve the accuracy of discrimination of fire from non-fire conditions. Thermal sensing technologies can also combined with smoke sensors or with gas sensors to form a multi-criteria detector. Thermal sensors are not amenable to inexpensive and convenient self-testing.
The basic problems with combining a gas sensor with another sensor are relatively high costs and reliability issues with the gas sensors. Electrochemical gas sensors have problems with the detection of a sensor failure. Solid state gas sensors have problems with false sensing due to humidity and ambient temperature in addition to high current.
It would be desirable to be able to combine a relatively inexpensive alternate type of sensor with a smoke sensor to be able to enhance fire vs. non-fire discrimination. In addition, it would be desirable if such sensors could be inexpensively and conveniently tested.